Commercial Cu2Cr2O5 Decorated with Iron Carbide Nanoparticles as a Multifunctional Catalyst for Magnetically Induced Continuous‐Flow Hydrogenation of Aromatic Ketones

Abstract Copper chromite is decorated with iron carbide nanoparticles, producing a magnetically activatable multifunctional catalytic system. This system (ICNPs@Cu2Cr2O5) can reduce aromatic ketones to aromatic alcohols when exposed to magnetic induction. Under magnetic excitation, the ICNPs generate locally confined hot spots, selectively activating the Cu2Cr2O5 surface while the global temperature remains low (≈80 °C). The catalyst selectively hydrogenates a scope of benzylic and non‐benzylic ketones under mild conditions (3 bar H2, heptane), while ICNPs@Cu2Cr2O5 or Cu2Cr2O5 are inactive when the same global temperature is adjusted by conventional heating. A flow reactor is presented that allows the use of magnetic induction for continuous‐flow hydrogenation at elevated pressure. The excellent catalytic properties of ICNPs@Cu2Cr2O5 for the hydrogenation of biomass‐derived furfuralacetone are conserved for at least 17 h on stream, demonstrating for the first time the application of a magnetically heated catalyst to a continuously operated hydrogenation reaction in the liquid phase.


General
Experiments involving magnetic induction and high-pressure of H2 must be carried out only with appropriate equipment and under rigorous safety precautions.

Methods
All synthetic procedures were carried out under argon using Schlenk techniques or the glovebox. Mesitylene (99%), toluene (99%), and tetrahydrofuran (99%) were purchased from VWR, heptane (99%) from Carl Roth and all solvents were degassed under argon and dried over molecular sieves (4 Å) before use. The commercial products, hexadecylamine (HDA), nanoparticles were shown to be of spherical or cubic shape with an average diameter of 12 nm ( Figure S1a). Carbidization leads to an increase in particle size to about 14 nm for the ICNPs due to the incorporation of carbon ( Figure S1b), which is in good agreement with the previously published results. 2 XRD (X-ray diffraction) measurements were performed on a PANalytical Empyrean diffractometer using Co-Kα radiation (λ=0.1789 nm) at 45 kV and 40 mA. The XRD patterns obtained for Fe(0) and ICNPs compare well to those reported 2 and confirm the presence of an iron carbide phase after carbidization ( Figure S2). Magnetic measurements were performed on a VSM (Vibrating Sample Magnetometer, Quantum Device PPMS Evercool II). Plotting sample magnetization against applied field strength gives a saturation magnetization of ≈150 emu/g ( Figure S3), comparable to the previously reported values. 2 SAR (Specific absorption rate) measurements to further determine the heating power of the ICNPs were conducted using the previously published calorimeter set-up. The SAR values obtained in correlation with the applied field strength are in accordance with those reported previously ( Figure S4). 2 For all XRD, VSM and SAR measurements air-tight sample holders were used and prepared in the glovebox. The iron content of ICNPs was determined by TGA (thermogravimetric analysis). TGA measurements were performed on a Mettler Toledo thermogravimetric analyzer using oxidation-reduction with a temperature ramp to 700°C to remove all carbon content from the sample. The samples were shown to contain ≈ 75 wt% iron, which is in good agreement with the previously published values. 2 The size and the morphology of ICNPs@Cu2Cr2O5 were studied by SEM (scanning electron microscopy) instead of TEM due to its better surface sensitivity and to avoid poor image contrast between the heavy elements (Fe,Cu,Cr  Figure S5). The observations were in accordance with those in literature. 3,4 Further, XRD spectra of ICNPs@Cu2Cr2O5 were recorded in comparison to pure Cu2Cr2O5, but unfortunately there was no obvious difference between the spectra ( Figure S6), most likely due to the overlapping peak regions of Fe/Fe2.2C with Cu2Cr2O5 and the relatively low loading of ICNPs.
Product analysis was done by GC-FID (gas chromatography coupled with flame ionization detection) on a Shimadzu GC 2030 equipped with a CP-WAX-52CB column and further by GC-MS (gas chromatography coupled with mass spectrometry) on a Shimadzu QP 2020 instrument. Samples obtained from the reaction mixture as described above were injected into the GC-FID and were identified according to the respective product retention times. Product quantification was done by referencing the product peak area to the peak area of the added tetradecane standard, following internal GC calibration with the isolated products. For identification of unknown products and trace compounds GC-MS was used with its internal compound library for product identification.

Iron carbide nanoparticles (ICNPs)
ICNPs were synthesized according to the previously published procedure. 2

Continuous flow miniplant
The continuous flow miniplant with magnetic induction heating consists of a high pressure HPCL glass column as a tube reactor, PEEK and stainless steel tubing, a HPLC pump, and a series of pressure and temperature transducers, as shown in Figure S7. HPLC pump supplies the organic feed. H2 is dosed into the system via a massflow control. Organic reagents and H2 are mixed in a Tee piece, then the fluid flows through the glass tube reactor where it is subjected to electromagnetic field. For comparable experiments using conventional heating method, the mixed fluid is preheated by trace heating, and then heated by trace heating in the reactor as well. The system pressure is controlled at its outlet using a BPR and the product is collected at this point.

Continuous flow experiments
In a typical experiment, ICNPs@Cu2Cr2O5 (500 mg) and glass beads (1 mm  mL.min -1 ) that enters the catalyst bed in form of bubbles prior to magnetic field in the glass tube reactor. Once the system was stable, the alternating magnetic field started. The global temperature was monitored by an infrared camera. The crude products was directly analyzed by on-line GC over time.
For these experiments, a large excess of H2 was used in order to avoid having a H2 concentration gradient along the reactor bed. As a result, a full conversion of substrate 1 to 1c corresponds to a H2 conversion of ca. 3%. This is also very similar to the excess of H2 used in the batch reactions.
Determination of the liquid residence time in the catalyst bed: The reactor was packed with Cu2Cr2O5 and glass beads in the same way as for the catalytic         Reactions performed in Fischer-Porter-Bottle, with 0.1 mmol furfuralacetone in 0.5 mL heptane under 3 bar H2 without stirring for 3 h. Catalyst quantities used were a) 30 mg ICNPs@Cu2Cr2O5 (containing respective amounts of 3.5 mg ICNPs and 26.5 mg Cu2Cr2O5), b) 3.5 mg ICNPs, c) 26.5 mg Cu2Cr2O5. 1c = furfuryl alcohol from conversion of furfuralacetone. Classical heating: 120°C. Magnetic heating: 64 mT, 350 kHz.